The Precise Moment: Tempered Glass

All About Glass

Glass breaks. But if it’s strengthened by thermal tempering, it breaks less easily and more safely. By 1920, architects and European car designers wanted more and more tempered glass—and in large sheets. Glassmakers could successfully temper only one sheet in ten.

There’s a trick to tempering: heat the glass until it almost softens and, at that precise moment, cool the surface rapidly and uniformly. Heat the glass too much and it becomes distorted, too little and it doesn’t temper. Cool it unevenly and it breaks.

Achille Verlay, who worked at French glassmaker Saint-Gobain, came up with a clever tempering solution in 1929. Using tongs with the tips of tungsten steel, he hung a glass sheet in an oven. At the right temperature, the tips glowed, signaling that the glass was ready for cooling. Jets of air were then blasted uniformly on both sides of the glass, instantly tempering it. The process was nearly foolproof. It was automated and used for most of the 20th century to temper everything from car windows to basketball backboards.

The Curious Glass Drops

Prince Rupert of Bohemia (1619-1682) brought some curious glass drops from the Continent to England in 1660. They had been made by dripping molten glass in cold water. The tadpole-shaped drops could be hammered repeatedly on the heads without breaking. Why were they so strong?

The drops remained a curiosity until, almost 200 years later, scientists discovered what was happening: when hot glass is quickly chilled, the fast-cooling outside layer is compressed. The slower-cooling inside is under tension. The only way to break the glass is to penetrate the tough outer layer and release the tension.

The head of a Prince Rupert drop is tough, but the tail is thin and can be broken. When the tail is snapped, all the tension trapped inside the drop is released, and the entire drop explodes into a fine glass powder.

In the 1920s, Corning physicist Jesse Littleton experimented to find the best temperatures for tempering glass and maximizing its strength. His discoveries gave glassmakers control over the tempering process and laid the ground-work for the flat-glass tempering method later developed by Saint-Gobain.

Even Stronger: Chemically Tempered Glass

The windows of this space capsule have inner panes of chemically tempered glass for extra safety. Chem-tempered glass is 25 times as strong as ordinary glass. It’s made by a chemical process that—like thermal strengthening—makes glass strong by compressing its surface.

The glass is placed in a bath of molten potassium salts. The combination of heat and salts causes the large potassium atoms to exchange places with smaller sodium atoms in the glass. As the surface of the glass becomes crowded, it becomes compressed.

The Corning Museum of Glass
This article was originally published in Innovations in Glass, 1999, pp. 16–18.